1. Field of the Invention
This invention relates to the field of polymer chemistry. More particularly, this invention relates to the free radical cure of a thermally stable polymer. Still more particularly, but without limitation thereto, this invention relates to the free radical cure of the bismaleimide of dimer diamine with hydroperoxides.
2. Description of the Prior Art
Polymers with improved heat resistance are in constant demand for use in composites, adhesives and coatings in missiles and aircraft. A critical requirement for polymers with these intended uses is typically prolonged exposure to temperatures up to 230.degree. C. (450.degree. F.) without significant degradation.
Aromatic substituted bismaleimide prepolymers have been considered because of their exceptional thermal stability after cure. Prepared from parent aromatic diamines and maleic anhydride, these prepolymers are usually solids at room temperature and commonly can be cured above their melting points at temperatures greater than 200.degree. C. Because they are solids, these aromatic prepolymers require a solvent or must be heated to the melt temperature to be used.
For processing and fabrication purposes, it would be advantageous to cure bismaleimides without using a solvent and at lower temperatures. Application with solvents can be detrimental to the properties of the cured polymers since solvent molecules may remain trapped in the interstitial volume of the polymeric network. Attaining lower cure temperatures would result in less costly processes with decreased energy demands.
Aliphatic substituted bismaleimide "monomers" or prepolymers are a superior substitute for the aromatic systems since selected moderate molecular weight liquid diamines would yield liquid bismaleimides. Alternate cures, instead of thermal, of these materials could be accomplished at lower temperatures. Furthermore, the need for a solvent to facilitate processing these materials to thermally stable polymers can be eliminated so that structural integrity of the polymers can be maintained.